Broaching of work pieces having helical teeth by broaches designed to finish cut the sides of the teeth is well known.
In prior U.S. Pat. No. 2,987,801 there is disclosed a roughing broach, having a floating shell mounted on its trailing portion. The floating shell has laterally stepped teeth which perform a side shaving operation. The disclosure suggests that both spur and helical work pieces can be produced.
While theoretically the structure disclosed in U.S. Pat. No. 2,987,801 would produce accurate helically toothed work pieces, it was found that when producing helically toothed work pieces, errors appeared in lead, or as tooth distortions, due largely to the fact that with helical broach teeth, the cutting action and guiding action is different at the sides forming acute and obtuse included angles with the front tooth surface. This led to further developments, and U.S. Pat. No. 3,276,100 disclosed a finishing shell specifically modified for broaching helical teeth, in which a series of helically aligned broach teeth was divided into leading and following groups, each group of teeth being non-cutting on one side and cutting only on the other with the relationship reversed in the two sections.
This represented a substantial advance in the art, but as ever higher standards of accuracy were imposed, it became apparent that further refinements were required. Other variations as disclosed in U.S. Pat. Nos. 3,178,800, 3,199,172, 3,217,383, 3,231,962, 3,267,552 and 3,276,099, were used and in particular cases solved special problems. However, no single broach design was found in all cases to produce the highly accurate helical teeth required by present technology.
The present invention relates to an improvement over prior U.S. Pat. No. 3,276,100 which improvement in production broaches has proved to eliminate errors previously requiring in many cases specific special modifications to overcome specific tooth errors.
In accordance with the present invention, a finishing broach or broach section for broaching helical teeth has a multiplicity of series of helically aligned teeth. All of the teeth have generally helically inclined sides which intersect the front faces to provide acute and obtuse included angles at opposite sides of the teeth. Also all of the teeth are shaped to conform to the profile of the tooth space in which they operate.
The teeth of each series are divided generally into a leading section and a following section.
All of the teeth in the leading section at the acute included angle sides thereof are laterally stepped and backed off to take a progression of shaving cuts on the adjacent tooth surface for the full tooth height. In this leading section, the teeth at the obtuse side have neither backoff nor lateral stepping.
The teeth of the following section at the obtuse side are laterally stepped and backed off throughout. At the acute side of the teeth in the trailing section, the first few teeth are not stepped laterally, but are backed off. Following these few teeth there is provided a single tooth which at the acute side is neither stepped nor backed off, but is provided with a side chamfer extending from its front face rearwardly. Following this single tooth the remaining teeth in the trailing section at the acute side are neither stepped nor backed off.